In high frequency surgery (HF surgery), alternating currents are passed through the human body at high frequency to damage or cut tissues in a targeted way. A substantial advantage over conventional scalpel cutting techniques is provided because suppression of bleeding can take place simultaneously with the incision by closing the relevant vessels.
A monopolar technique is often used. With such a technique, one pole of the HF voltage source is connected to the patient over the largest area possible. This electrode is known as the neutral electrode. The other pole (the active electrode) is situated on a surgical instrument. The current flows from the active electrode to the neutral electrode. The current density is highest in the immediate vicinity of the active electrode and this is where coagulating or parting of the tissue takes place.
When using neutral electrodes, one must ensure that the contact resistance between the skin and the contacting electrode is not too high. This would lead to severe heating of the biological tissue and occasionally to burns. Recently, the problem has presented itself that more methods have been developed by which relatively large HF currents are applied over longer periods of time. The risk of a burn at the neutral electrode is therefore increased. It should also be noted that, due to the physical conditions, the maximum heating occurs at the edge regions of the neutral electrode. The risk of burning is therefore particularly high in these edge regions.
Large area neutral electrodes are used to prevent unwanted damage to the tissue by contributing to reducing the current density in the immediate vicinity of the neutral electrode. Monitoring devices, which recognize partial detachment of the neutral electrode and react to this event accordingly, also exist.
Currently, applicable standards prescribe tests that limit a temperature rise at the neutral electrode, upon application of a particular current over a predetermined time period, to a maximum value.
With the solutions that are conventionally selected, a further problem exists wherein the area of the neutral electrode cannot be increased without restriction, otherwise the application would no longer be practical. Suitable placement of the neutral electrode becomes more difficult with its increasing size. Furthermore, in pediatric surgery, narrow limits apply to the size of the electrode.
The monitoring devices known and used today can only indirectly determine the degree of risk because it is usually only the electrical contact resistance between the neutral electrode and the patient that is measured, which only has a vague correlation to the application area.
As mentioned above, the current densities at the neutral electrode are not evenly distributed. For example, severe heating leading to burning can occur at the edges. Monitoring these local effects is extremely difficult.